Readme tweaks

I found initial setup with a bit confusing. This is a refactoring of the README to try to improve this:

- fixes some typos
- makes a clear installation section & moves important notes from other parts of the doc into it

My editor is configured to auto-remove trailing whitespace, so some of the diff is simply that. If these changes are acceptable I may refactor a bit more to make the steps to getting started a bit clearer as well.

Thanks for an awesome tool!